french onion chicken

Hall of Fame!  This chicken met every expectation I had in my mind when I saw this recipe. Definitely "guest worthy" and if I weren't planning on making Chicken Cordon Bleu for Valentine's Day I'd make this.

french onion chicken (for the love of cooking) 
2 tablespoons butter (I used 1 tablespoon)
2 large sweet yellow onions, sliced
2 cloves garlic, minced
1 teaspoon fresh thyme leaves plus more for garnishing
2 tablespoons flour
2 cups beef broth
2 teaspoons olive oil (I omitted because I have nonstick pans)
3 boneless, skinless chicken breasts, cut in half lengthwise
sea salt and pepper to taste
garlic powder to taste
1 tablespoon balsamic vinegar
2 tablespoons Dijon mustard
1/2- 1 cup Greyere cheese, grated

Melt the butter in an oven-proof pan over medium heat, add the onions and cook until deeply golden brown, about an hour, stirring often, adding a bit of beef broth to deglaze the pan as needed.
Preheat the oven to 350 degrees.
Add the garlic, thyme and flour to the onions then cook for another minute, stirring constantly.  Add one cup of the beef broth, deglaze the pan making sure to scrape up any browned bits on the bottom of the pan, bring to a boil, reduce heat and simmer 5 minutes.
Meanwhile, heat another pan with the oil.  Cut each chicken breast and season with sea salt, pepper and garlic powder to taste.  Add the chicken to the hot pan and brown on both sides, about 3-4 minutes per side, set aside on a plate.
Add the remaining cup of broth to the pan that you cooked the chicken in, deglaze it making sure to scrape up any browned bits off the bottom of the pan, whisk in the balsamic vinegar and mustard, bring it to a boil, then reduce the heat and simmer to reduce the sauce by half, about 5 minutes.
Pour the vinegar/mustard sauce on the onions and mix well to combine.  Add the chicken and its juices to the onions then spoon sauce over each piece.  Bake in the oven for 10 minutes.
Remove the chicken from the oven and sprinkle the top with cheese.  Place back into the oven until the cheese has melted.  Sprinkle the top with fresh thyme leaves, if desired, and serve.